Single on Twelve HTTP status codes, by the book

RFC 9110 (HTTP Semantics, https://www.rfc-editor.org/rfc/rfc9110.html) is the current specification, and it answers every column for all 12 codes. Of the 12, seven are heuristically cacheable: 203, 206, 308, 405, 410, 414 and 501. The other five (303, 307, 413, 421, 422) are not.

How cacheability was decided. Section 15.1 of the RFC lists the codes it defines as heuristically cacheable: "200, 203, 204, 206, 300, 301, 308, 404, 405, 410, 414, and 501 in this specification". It then says "all other status codes are not heuristically cacheable." I checked each code's own section against that list:

• Cacheable: the sections for 203, 206, 308, 405, 410, 414 and 501 each include an "A NNN response is heuristically cacheable" sentence.
• 206 differs slightly: its sentence leaves out "the method definition". It reads "unless otherwise indicated by explicit cache controls".
• Not cacheable: the sections for 303, 307, 413, 421 and 422 have no such sentence.

Renamed codes. Two codes have different names in RFC 9110 than in older documents:

• 413 is now "Content Too Large" (it was "Payload Too Large").
• 422 is now "Unprocessable Content" (it was "Unprocessable Entity").

I used the RFC 9110 names, since those are the current ones.

Quotes. Every quote comes from the code's own numbered section in Section 15. The page wraps text across lines; in the quotes those line breaks are written as single spaces.

{"answers":[
{"code":"203","reason":"Non-Authoritative Information","section":"15.3.4","heuristicallyCacheable":true,"quote":"This status code allows the proxy to notify recipients when a transformation has been applied, since that knowledge might impact later decisions regarding the content.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"206","reason":"Partial Content","section":"15.3.7","heuristicallyCacheable":true,"quote":"The 206 (Partial Content) status code indicates that the server is successfully fulfilling a range request for the target resource by transferring one or more parts of the selected representation.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"303","reason":"See Other","section":"15.4.4","heuristicallyCacheable":false,"quote":"This status code is applicable to any HTTP method.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"307","reason":"Temporary Redirect","section":"15.4.8","heuristicallyCacheable":false,"quote":"Since the redirection can change over time, the client ought to continue using the original target URI for future requests.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"308","reason":"Permanent Redirect","section":"15.4.9","heuristicallyCacheable":true,"quote":"The 308 (Permanent Redirect) status code indicates that the target resource has been assigned a new permanent URI and any future references to this resource ought to use one of the enclosed URIs.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"405","reason":"Method Not Allowed","section":"15.5.6","heuristicallyCacheable":true,"quote":"The origin server MUST generate an Allow header field in a 405 response containing a list of the target resource's currently supported methods.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"410","reason":"Gone","section":"15.5.11","heuristicallyCacheable":true,"quote":"The 410 (Gone) status code indicates that access to the target resource is no longer available at the origin server and that this condition is likely to be permanent.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"413","reason":"Content Too Large","section":"15.5.14","heuristicallyCacheable":false,"quote":"The 413 (Content Too Large) status code indicates that the server is refusing to process a request because the request content is larger than the server is willing or able to process.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"414","reason":"URI Too Long","section":"15.5.15","heuristicallyCacheable":true,"quote":"The 414 (URI Too Long) status code indicates that the server is refusing to service the request because the target URI is longer than the server is willing to interpret.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"421","reason":"Misdirected Request","section":"15.5.20","heuristicallyCacheable":false,"quote":"A proxy MUST NOT generate a 421 response.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"422","reason":"Unprocessable Content","section":"15.5.21","heuristicallyCacheable":false,"quote":"For example, this status code can be sent if an XML request content contains well-formed (i.e., syntactically correct), but semantically erroneous XML instructions.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"},
{"code":"501","reason":"Not Implemented","section":"15.6.2","heuristicallyCacheable":true,"quote":"This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.","url":"https://www.rfc-editor.org/rfc/rfc9110.html"}
]}